Instability mitigation system using stator plasma actuators

ABSTRACT

An instability mitigation system is disclosed, comprising a stator stage located axially proximate to a rotor, the stator stage having a row of a plurality of stator vanes arranged around a centerline axis, and a mitigation system comprising at least one plasma actuator mounted on the stator vane that facilitates the improvement of the stability of the rotor, and a control system for controlling the operation of the mitigation system. An instability mitigation system further comprising a detection system for detecting an onset of an instability in a rotor and a control system for controlling the detection system and the mitigation system are disclosed.

BACKGROUND OF THE INVENTION

This invention relates generally to gas turbine engines, and, morespecifically, to a system for detection of an instability such as astall in a compression system such as a fan or a compressor used in agas turbine engine.

In a turbofan aircraft gas turbine engine, air is pressurized in acompression system, comprising a fan module, a booster module and acompression module during operation. In large turbo fan engines, the airpassing through the fan module is mostly passed into a by-pass streamand used for generating the bulk of the thrust needed for propelling anaircraft in flight. The air channeled through the booster module andcompression module is mixed with fuel in a combustor and ignited,generating hot combustion gases which flow through turbine stages thatextract energy therefrom for powering the fan, booster and compressorrotors. The fan, booster and compressor modules have a series of rotorstages and stator stages. The fan and booster rotors are typicallydriven by a low pressure turbine and the compressor rotor is driven by ahigh pressure turbine. The fan and booster rotors are aerodynamicallycoupled to the compressor rotor although these normally operate atdifferent mechanical speeds.

Operability in a wide range of operating conditions is a fundamentalrequirement in the design of compression systems, such as fans, boostersand compressors. Modern developments in advanced aircrafts have requiredthe use of engines buried within the airframe, with air flowing into theengines through inlets that have unique geometries that cause severedistortions in the inlet airflow. Some of these engines may also have afixed area exhaust nozzle, which limits the operability of theseengines. Fundamental in the design of these compression systems isefficiency in compressing the air with sufficient stall margin over theentire flight envelope of operation from takeoff, cruise, and landing.However, compression efficiency and stall margin are normally inverselyrelated with increasing efficiency typically corresponding with adecrease in stall margin. The conflicting requirements of stall marginand efficiency are particularly demanding in high performance jetengines that operate under challenging operating conditions such assevere inlet distortions, fixed area nozzles and increased auxiliarypower extractions, while still requiring high a level of stabilitymargin throughout the flight envelope.

Instabilities, such as stalls, are commonly caused by flow breakdowns onthe airfoils of the rotor blades and stator vanes of compression systemssuch as fans, compressors and boosters. In gas turbine enginecompression system rotors, there are tip clearances between rotatingblade tips and a stationary casing or shroud that surrounds the bladetips. During the engine operation, air leaks from the pressure side of ablade through the tip clearance toward the suction side. These leakageflows may cause vortices to form at the tip region of the blade. A tipvortex can grow and spread in the spanwise and chordwise directions onthe rotor blades and stator vanes. Flow separations on the stator androtor airfoils may occur when there are severe inlet distortions in theair flowing into compression system, or when the engine is throttled,and lead to a compressor stall and cause significant operabilityproblems and performance losses.

Accordingly, it would be desirable to have the ability to measure andcontrol dynamic processes such as flow instabilities in compressionsystems. It would be desirable to have a detection system that canmeasure a compression system parameter related to the onset of flowinstabilities, such as the dynamic pressure near the blade tips or otherlocations, and process the measured data to detect the onset of aninstability such as a stall in compression systems, such as fans,boosters and compressors. It would be desirable to have a mitigationsystem to mitigate compression system instabilities based on thedetection system output, for certain flight maneuvers at critical pointsin the flight envelope, allowing the maneuvers to be completed withoutinstabilities such as stalls and surges. It would be desirable to havean instability mitigation system that can control and manage thedetection system and the mitigation system.

BRIEF DESCRIPTION OF THE INVENTION

The above-mentioned need or needs may be met by exemplary embodimentswhich provide a compression system the compression system comprising astator stage having a circumferential row of stator vanes having a vaneairfoil, a rotor having a circumferential row of blades, each bladehaving a blade airfoil, wherein stator stage is located axially forwardor aft of the rotor, a detection system for detecting an instability inthe rotor during operation, a mitigation system that facilitates theimprovement of the stability of compression system when an instabilityis detected, and a control system for controlling the operation of themitigation system.

In one exemplary embodiment, a gas turbine engine comprising a fansection, a detection system for detecting an instability during theoperation of the fan section and a mitigation system that facilitatesthe improvement of the stability of the fan section is disclosed.

In another exemplary embodiment, a detection system is disclosed fordetecting onset of an instability in a multi-stage compression systemrotor comprising a pressure sensor located on a casing surrounding tipsof a row of rotor blades wherein the pressure sensor is capable ofgenerating an input signal corresponding to the dynamic pressure at alocation near the rotor blade tip.

In another exemplary embodiment, a mitigation system is provided tomitigate compression system instabilities for increasing the stableoperating range of a compression system, the system comprising at leastone plasma generator located on a stator stage of the compressionsystem. The plasma generator comprises a first electrode and a secondelectrode separated by a dielectric material. The plasma generator isoperable for forming a plasma between first electrode and the secondelectrode.

In another exemplary embodiment, the plasma actuator is mounted on thestator airfoil in a generally spanwise direction. In another exemplaryembodiment the plasma actuator system comprises a plasma actuatormounted on a movable flap of an inlet guide vane.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the concluding part of thespecification. The invention, however, may be best understood byreference to the following description taken in conjunction with theaccompanying drawing figures in which:

FIG. 1 is a schematic cross-sectional view of a gas turbine engine withan exemplary embodiment of the present invention.

FIG. 2 is an enlarged cross-sectional view of a portion of the fansection of the gas turbine engine shown in FIG. 1, showing an exemplaryembodiment of plasma actuators mounted on stator airfoils.

FIG. 3 is an exemplary operating map of a compression system in the gasturbine engine shown in FIG. 1.

FIG. 4 is a schematic cross sectional view of an exemplary embodiment ofthe present invention showing an exemplary detection system mounted on astatic component.

FIG. 5 is a schematic illustration of a mitigation system with a plasmaactuator illustrated in FIG. 2 energized.

FIG. 6 shows two stator stages having an exemplary arrangement of plasmaactuators and a detection system mounted in a static component nearrotor blade tip region.

FIG. 7 is a cross sectional view of a stator airfoil having an exemplaryarrangement of multiple plasma actuators mounted on the convex side.

FIG. 8 is an isometric view of a stator vane having an exemplaryarrangement of a plasma actuator mounted in a spanwise direction nearthe stator airfoil leading edge.

FIG. 9 is a schematic sketch of an exemplary embodiment of aninstability mitigation system showing an exemplary arrangement ofmultiple sensors mounted on a casing and plasma actuators mounted on astator stage.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings wherein identical reference numerals denotethe same elements throughout the various views, FIG. 1 shows anexemplary turbofan gas turbine engine 10 incorporating an exemplaryembodiment of the present invention. It comprises an engine centerlineaxis 8, fan section 12 which receives ambient air 14, a high pressurecompressor (HPC) 18, a combustor 20 which mixes fuel with the airpressurized by the HPC 18 for generating combustion gases or gas flowwhich flows downstream through a high pressure turbine (HPT) 22, and alow pressure turbine (LPT) 24 from which the combustion gases aredischarged from the engine 10. Many engines have a booster or lowpressure compressor (not shown in FIG. 1) mounted between the fansection and the HPC. A portion of the air passing through the fansection 12 is bypassed around the high pressure compressor 18 through abypass duct 21 having an entrance or splitter 23 between the fan section12 and the high pressure compressor 18. The HPT 22 is joined to the HPC18 to substantially form a high pressure rotor 29. A low pressure shaft28 joins the LPT 24 to the fan section 12 and the booster if one isused. The second or low pressure shaft 28 is rotatably disposedco-axially with and radially inwardly of the first or high pressurerotor. In the exemplary embodiments of the present invention shown inFIGS. 1 and 2, the fan section 12 has a multi-stage fan rotor, as inmany gas turbine engines, illustrated by first, second, and third fanrotor stages 12 a, 12 b, and 12 c respectively, and a plurality ofstator stages 31, each stator stage having a circumferential row ofstator vanes such as 31 a, 31 b and 31 c. Each stator stage is locatedin axial fwd or aft from a rotor such as 12 a. For example, as shown inFIG. 2, the stator stage having a circumferential row of stator vanes 31a is located axially aft from the rotor 12 a. It is common to have acircumferential row of Inlet guide vanes (IGV) at the inlet to thecompression system, as shown in FIG. 2. The IGV's may have movable flaps32, located on its aft end, as shown in FIG. 2.

The fan section 12 that pressurizes the air flowing through it isaxisymmetrical about the longitudinal centerline axis 8. The fan section12 shown in FIG. 2 includes a plurality of inlet guide vanes (IGV) 30and a plurality of stator vanes 31 a, 31 b, 31 c arranged in acircumferential direction around the longitudinal centerline axis 8. Themultiple rotor stages 12 a, 12 b, 12 c of the fan section 12 havecorresponding fan rotor blades 40 a, 40 b, 40 c extending radiallyoutwardly from corresponding rotor hubs 39 a, 39 b, 39 c in the form ofseparate disks, or integral blisks, or annular drums in any conventionalmanner.

Cooperating with a fan rotor stage 12 a, 12 b, 12 c shown in FIG. 2 is acorresponding stator stage 31 comprising a plurality ofcircumferentially spaced apart stator vanes 31 a, 31 b, 31 c. Anexemplary arrangement of stator vanes and rotor blades is shown in FIG.2. The rotor blades 40 and stator vanes 31 a, 31 b, 31 c have airfoilshaving corresponding aerodynamic profiles or contours for pressurizingthe airflow successively in axial stages. Each fan rotor blade 40comprises an airfoil 34 extending radially outward from a blade root 45to a blade tip 46, a concave side (also referred to as “pressure side”)43, a convex side (also referred to as “suction side”) 44, a leadingedge 41 and a trailing edge 42. The airfoil 34 extends in the chordwisedirection between the leading edge 41 and the trailing edge 42. A chordC of the airfoil 34 is the length between the leading 41 and trailingedge 42 at each radial cross section of the blade. The pressure side 43of the airfoil 34 faces in the general direction of rotation of the fanrotors and the suction side 44 is on the other side of the airfoil.

A stator stage 31 is located in axial proximity to a rotor, such as forexample item 12 b. Each stator vane, such as shown as items 31 a, 31 b,31 c in FIG. 2, in a in a stator stage 31 comprises an airfoil 35extending radially in a generally spanwise direction corresponding tothe span between the blade root 45 and the blade tip 46. Each statorvane, such as item 31 a, has a vane concave side (also referred to as“pressure side”) 57, a vane convex side (also referred to as “suctionside”) 58, a vane leading edge 36 and a vane trailing edge 37. The vaneairfoil 35 extends in the chordwise direction between the leading edge36 and the trailing edge 37. A chord of the airfoil 35 is the lengthbetween the leading 36 and trailing edge 37 at each radial cross sectionof the stator vane. At the front of the compression system, such as thefan section 12, is a stator stage having a set if inlet guide vanes 30(“IGV”) that receive the airflow into the compression system. The inletguide vanes 30 have a suitably shaped aerodynamic profile to guide theairflow into the first stage rotor 12 a. In order to suitably orient theairflow into the compression system, the inlet guide vanes 30 may haveIGV flaps 32 that are moveable, located near their aft end. The IGV flap32 is shown in FIG. 2 at the aft end of the IGV 30. It is supportedbetween two hinges at the radially inner end and the outer end such thatit is can be moved during the operation of the compression system.

The rotor blades rotate within a static structure, such as a casing or ashroud, that are located radially apart from and surrounding the bladetips, as shown in FIG. 2. The front stage rotor blades 40 rotate withinan annular casing 50 that surrounds the rotor blade tips. The aft stagerotor blades of a multi stage compression system, such as the highpressure compressor shown as item 18 in FIG. 1, typically rotate withinan annular passage formed by shroud segments 51 that arecircumferentially arranged around the blade tips 46. In operation,pressure of the air is increased as the air decelerates and diffusesthrough the stator and rotor airfoils.

Operating map of an exemplary compression system, such as the fansection 12 in the exemplary gas turbine engine 10 is shown in FIG. 3,with inlet corrected flow rate along the horizontal axis and thepressure ratio on the vertical axis. Exemplary operating lines 114, 116and the stall line 112 are shown, along with exemplary constant speedlines 122, 124. Line 124 represents a lower speed line and line 122represents a higher speed line. As the compression system is throttledat a constant speed, such as constant speed line 124, the inletcorrected flow rate decreases while the pressure ratio increases, andthe compression system operation moves closer to the stall line 112.Each operating condition has a corresponding compression systemefficiency, conventionally defined as the ratio of ideal (isentropic)compressor work input to actual work input required to achieve a givenpressure ratio. The compressor efficiency of each operating condition isplotted on the operating map in the form of contours of constantefficiency, such as items 118, 120 shown in FIG. 3. The performance maphas a region of peak efficiency, depicted in FIG. 3 as the smallestcontour 120, and it is desirable to operate the compression systems inthe region of peak efficiency as much as possible. Flow distortions inthe inlet air flow 14 which enters the fan section 12 tend to cause flowinstabilities as the air is compressed by the fan blades (andcompression system blades) and the stall line 112 will tend to droplower. As explained further below herein, the exemplary embodiments ofthe present invention provide a system for detecting the flowinstabilities in the fan section 12, such as from flow distortions, andprocessing the information from the fan section to predict an impendingstall in a fan rotor. The embodiments of the present invention shownherein enable other systems in the engine which can respond as necessaryto manage the stall margin of fan rotors and other compression systemsby raising the stall line, as represented by item 113 in FIG. 3.

Stalls in fan rotors due to inlet flow distortions, and stalls in othercompression systems that are throttled, are known to be caused by abreakdown of flow or flow separation in the stator and rotor airfoils,especially near the tip region 52 of rotors, such as the fan rotors 12a, 12 b, 12 c shown in FIG. 2. Flow breakdown near blade tips isassociated with tip leakage vortex that has negative axial velocity,that is, the flow in this region is counter to the main body of flow andis highly undesirable. Unless interrupted, the tip vortex propagatesaxially aft and tangentially from the blade suction surface 44 to theadjacent blade pressure surface 43. As the inlet flow distortions becomesevere, or as a compression system is throttled, the blockage becomesincreasingly larger within the flow passage between the adjacent bladesand vanes and eventually becomes so large as to drop the rotor pressureratio below its design level, and causes the compression system tostall.

The ability to control a dynamic process, such as a flow instability ina compression system, requires a measurement of a characteristic of theprocess using a continuous measurement method or using samples ofsufficient number of discrete measurements. In order to mitigate fanstalls for certain flight maneuvers at critical points in the flightenvelope where the stability margin is small or negative, a flowparameter in the engine is first measured that can be used directly or,with some additional processing, to predict the onset of stall of astage of a multistage fan shown in FIG. 2.

FIG. 4 shows an exemplary embodiment of a system 500 for detecting theonset of an aerodynamic instability, such as a stall or surge, in acompression stage in a gas turbine engine 10. In the exemplaryembodiment shown in FIG. 2, a fan section 12 is shown, comprising athree stage fan having rotors, 12 a, 12 b and 12 c and stator stageshaving stator vanes 31 a, 31 b, 31 c, and IGVs 30. The embodiments ofthe present invention can also be used in a single stage fan, or inother compression systems in a gas turbine engine, such as a highpressure compressor 18 or a low pressure compressor or a booster. In theexemplary embodiments shown herein, a pressure sensor 502 is used tomeasure the local dynamic pressure near the tip region 52 of the fanblade tips 46 during engine operation. Although a single sensor 502 canbe used for the flow parameter measurements, use of at least two sensors502 is preferred, because some sensors may become inoperable duringextended periods of engine operations. In the exemplary embodiment shownin FIG. 2, multiple pressure sensors 502 are used around the tips of fanrotors 12 a, 12 b, and 12 c.

In the exemplary embodiment shown in FIG. 4, the pressure sensor 502 islocated on a casing 50 that is spaced radially outwardly and apart fromthe fan blade tips 46. Alternatively, the pressure sensor 502 may belocated on a shroud 51 that is located radially outwardly and apart fromthe blade tips 46. The casing 50, or a plurality of shrouds 51,surrounds the tips of a row of blades 47. The pressure sensors 502 arearranged circumferentially on the casing 50 or the shrouds 51, as shownin FIG. 9. In an exemplary embodiment using multiple sensors on a rotorstage, the sensors 502 are arranged in substantially diametricallyopposite locations in the casing or shroud, as shown in FIG. 9.Alternatively, in other embodiments of the present invention, sensors502 may be mounted in locations in a stator stage 31 to measure flowparameters in the stator. Suitable sensors may also be mounted on thestator airfoil convex side 58 or concave side 57.

During engine operation, there is an effective clearance CL between thefan blade tip and the casing 50 or the shroud 51 (see FIG. 4). Thesensor 502 is capable of generating an input signal 504 in real timecorresponding to a flow parameter, such as the dynamic pressure in theblade tip region 52 near the blade tip 46. A suitable high responsetransducer, having a response capability higher than the blade passingfrequency is used. Typically these transducers have a responsecapability higher than 1000 Hz. In the exemplary embodiments shownherein the sensors 502 used were made by Kulite Semiconductor Products.The transducers have a diameter of about 0.1 inches and are about 0.375inches long. They have an output voltage of about 0.1 volts for apressure of about 50 pounds per square inch. Conventional signalconditioners are used to amplify the signal to about 10 volts. It ispreferable to use a high frequency sampling of the dynamic pressuremeasurement, such as for example, approximately ten times the bladepassing frequency.

The flow parameter measurement from the sensor 502 generates a signalthat is used as an input signal 504 by a correlation processor 510. Thecorrelation processor 510 also receives as input a fan rotor speedsignal 506 corresponding to the rotational speeds of the fan rotors 12a, 12 b, 12 c, as shown in FIGS. 1, 4 and 9. In the exemplaryembodiments shown herein, the fan rotor speed signal 506 is supplied byan engine control system 74, that is used in gas turbine engines.Alternatively, the fan rotor speed signal 506 may be supplied by adigital electronic control system or a Full Authority Digital ElectronicControl (FADEC) system used an aircraft engine.

The correlation processor 510 receives the input signal 504 from thesensor 502 and the rotor speed signal 506 from the control system 74 andgenerates a stability correlation signal 512 in real time usingconventional numerical methods. Auto correlation methods available inthe published literature may be used for this purpose. In the exemplaryembodiments shown herein, the correlation processor 510 algorithm usesthe existing speed signal from the engine control system 74 for cyclesynchronization. The correlation measure is computed for individualpressure transducers 502 over rotor blade tips 46 of the rotors 12 a, 12b, 12 c and input signals 504 a, 504 b, 504 c. The auto-correlationsystem in the exemplary embodiments described herein sampled a signalfrom a pressure sensor 502 at a frequency of 200 KHz. This relativelyhigh value of sampling frequency ensures that the data is sampled at arate at least ten times the fan blade 40 passage frequency. A window ofseventy two samples was used to calculate the auto-correlation having avalue of near unity along the operating line 116 and dropping towardszero when the operation approached the stall/surge line 112 (see FIG.3). For a particular fan stage 12 a, 12 b, 12 c when the stabilitymargin approaches zero, the particular fan stage is on the verge ofstall and the correlation measure is at a minimum. In the exemplaryinstability mitigation system 700 (see FIG. 9) disclosed herein designedto avoid an instability such as a stall or surge in a compressionsystem, when the correlation measure drops below a selected and pre-setthreshold level, an instability control system 600 receives thestability correlation signal 512 and sends an electrical signal 602 tothe engine control system 74, such as for example a FADEC system, and anelectrical signal 606 to an electronic controller 72, which in turn cantake corrective action using the available control devices to move theengine away from instability such as a stall or surge by raising thestall line as described herein. The methods used by the correlationprocessor 510 for gauging the aerodynamic stability level in theexemplary embodiments shown herein is described in the paper,“Development and Demonstration of a Stability Management System for GasTurbine Engines”, Proceedings of GT2006 ASME Turbo Expo 2006,GT2006-90324.

FIG. 4 shows schematically an exemplary embodiment of the presentinvention using a sensor 502 located in a casing 50 near the blade tipmid-chord of a blade 40. The sensor is located in the casing 50 suchthat it can measure the dynamic pressure of the air in the clearance 48between a fan blade tip 46 and the inner surface 53 of the casing 50. Inone exemplary embodiment, the sensor 502 is located in an annular groove54 in the casing 50. In other exemplary embodiments, it is possible tohave multiple annular grooves 54 in the casing 50, such as for example,to provide for tip flow modifications for stability. If multiple groovesare present, the pressure sensor 502 is located within one or more ofthese grooves, using the same principles and examples disclosed herein.Although the sensor is shown in FIG. 4 as located in a casing 50, inother embodiments, the pressure sensor 502 may be located in a shroud 51that is located radially outwards and apart from the blade tip 46. Thepressure sensor 502 may also be located in a casing 50 (or shroud 51)near the leading edge 41 tip or the trailing edge 42 tip of the blade40. The pressure sensor 502 may also be located in a stator stage 31 oron the stator vanes such as 31 a, 31 b, 31 c.

FIG. 9 shows schematically an exemplary embodiment of the presentinvention using a plurality of sensors 502 in a fan stage, such as item40 a in FIG. 2. The plurality of sensors 502 are arranged in the casing50 (or shroud 51) in a circumferential direction, such that pairs ofsensors 502 are located substantially diametrically opposite. Thecorrelations processor 510 receives input signals 504 from these pairsof sensors and processes signals from the pairs together. Thedifferences in the measured data from the diametrically opposite sensorsin a pair can be particularly useful in developing stability correlationsignal 512 to detect the onset of a fan stall due to engine inlet flowdistortions.

FIGS. 1, 6 and 9 show an exemplary embodiment of a mitigation system 300that facilitates the improvement of the stability of a compressionsystem when an instability is detected by the detection system 500 asdescribed previously. These exemplary embodiments of the invention useplasma actuators disclosed herein to reduce flow separation in statorvane airfoils 35 or rotor blade airfoils 34, and to delay the onset andgrowth of the blockage by the rotor blade tip leakage vortex describedpreviously herein. Plasma actuators used as shown in the exemplaryembodiments of the present invention, produce a stream of ions and abody force that act upon the fluid in the stator vane and rotor bladeairfoils, forcing it to pass through the blade passage in the directionof the desired fluid flow, reducing flow separations.

The terms “plasma actuators” and “plasma generators” as used herein havethe same meaning and are used interchangeably. FIG. 5 showsschematically, a plasma actuator 82, 84 illustrated in FIGS. 1, 2, 6, 7,8 and 9 when it is energized. The exemplary embodiment shown in FIG. 5shows a plasma generator 82 mounted to stator vane airfoil 31 a in astator stage 31, and includes a first electrode 62 and a secondelectrode 64 separated by a dielectric material 63. An AC (alternatingcurrent) power supply 70 is connected to the electrodes to supply a highvoltage AC potential in a range of about 3-20 kV to the electrodes 62,64. When the AC amplitude is large enough, the air ionizes in a regionof largest electric potential forming a plasma 68. The plasma 68generally begins near an edge 65 of the first electrode 62 which isexposed to the air and spreads out over an area 104 projected by thesecond electrode 64 which is covered by the dielectric material 63. Theplasma 68 (ionized air) in the presence of an electric field gradientproduces a force on the air flowing near the airfoils, inducing avirtual aerodynamic shape that causes a change in the pressuredistribution along the airfoil surfaces such that flow tends to remainattached to the airfoil surface, reducing flow separations. The air nearthe electrodes is weakly ionized, and usually there is little or noheating of the air.

FIG. 6 schematically illustrates, in cross-section view, exemplaryembodiment of a plasma actuator system 100 for improving the stabilityof compression systems and/or for enhancing the efficiency of acompression systems. The term “compression system” as used hereinincludes devices used for increasing the pressure of a fluid flowingthrough it, and includes the high pressure compressor 18, the boosterand the fan 12 used in gas turbine engines shown in FIG. 1. Theexemplary embodiments shown herein facilitate an increase in stallmargin and/or enhance the efficiency of compression systems in a gasturbine engine 10 such as the aircraft gas turbine engine illustrated incross-section in FIG. 1. The exemplary gas turbine engine plasmaactuator system 100 shown in FIG. 6 includes plasma generators 82mounted on stator vanes 31 a and 31 b. The plasma actuators shown inFIG. 6 are mounted in the stator vane airfoils 35 in a generallyspanwise direction, from the root to the tip of the airfoils. The plasmaactuators 82 are mounted in grooves located on the vane airfoil suctionside 58 such that the surfaces remain substantially smooth to avoiddisturbing local airflow near the plasma actuators. Suitable coveringusing conventional materials may be applied on the grooves after theplasma actuators are mounted to facilitate smooth airflow on the airfoilsurfaces. Each groove segment has the dielectric material 63 disposedwithin the groove segment separating the first electrodes 62 and secondelectrodes 64 disposed within the groove segments, forming the plasmaactuator 82. In another embodiment of the present invention, a pluralityof plasma actuators 82 are located on the convex side 58 of the statorvane airfoil 35. The plasma actuators are mounted at selected chordlengths from the leading edge 36, at locations selected based on thepropensity for airflow separation determined by conventional aerodynamicanalysis of airflow around the airfoil pressure and suction sides. Inanother embodiment of the present invention, plasma actuators may alsobe placed on the concave side 57 of the vane airfoil 35, especially nearthe trailing edge 37. FIG. 8 shows a stator vane having an exemplaryembodiment of the present invention wherein the plasma actuator 82 ismounted on the convex side of the vane airfoil, near the leading edge36, oriented in a generally span-wise direction. Alternately, it may beadvantageous to mount the plasma actuators at other orientations so asto align the plasma 68 direction along other suitable flow directions asdetermined by conventional aerodynamic analyses.

FIG. 9 shows schematically an exemplary embodiment of an instabilitymitigation system 700 according to the present invention. The exemplaryinstability mitigation system 700 comprises a detection system 500, amitigation system 300, a control system 74 for controlling the detectionsystem 500 and the mitigation system 300, including an instabilitycontrol system 600. The detection system 500, which has one or moresensors 502 to measure a flow parameter such as dynamic pressures nearblade tip, and a correlations processor 510, has been describedpreviously herein. The correlations processor 510 sends a correlationssignals 512 indicative of whether an onset of an instability such as astall has been detected at a particular rotor stage, or not, to theinstability control system 600, which in turn feeds back status signals604 to the control system 74. The control system 74 supplies informationsignals 506 related to the compression system operations, such as rotorspeeds, to the correlations processor 510. When an onset of aninstability is detected and the control system 74 determines that themitigation system 300 should be actuated, a command signal 602 is sentto the instability control system 600, which determines the location,type, extent, duration etc. of the instability mitigation actions to betaken and sends the corresponding instability control system signals 606to the electronic controller 72 for execution. The electronic controller72 controls the operations of the plasma actuator system 100 and thepower supply 70. These operations described above continue untilinstability mitigation is achieved as confirmed by the detection system500. The operations of the mitigation system 300 may also be terminatedat predetermined operating points determined by the control system 74.

In an exemplary instability mitigation system 700 system in a gasturbine engine 10 shown in FIG. 1, during engine operation, whencommanded by the instability control system 600 and an electroniccontroller 72, the plasma actuator system 100 turns on the plasmagenerator 82 (see FIGS. 6 and 9) to form the plasma 68 between the firstelectrode 62 and second electrode 64. The electronic controller 72 canalso be linked to an engine control system 74, such as for example aFull Authority Digital Electronic Control (FADEC), which controls thefan speeds, compressor and turbine speeds and fuel system of the engine.The electronic controller 72 is used to control the plasma generator 60by turning on and off of the plasma generator 60, or otherwisemodulating it as necessary to enhance the compression system stabilityby increasing the stall margin or enhancing the efficiency of thecompression system. The electronic controller 72 may also be used tocontrol the operation of the AC power supply 70 that is connected to theelectrodes to supply a high voltage AC potential to the electrodes.

In operation, when turned on, the plasma actuator system 100 produces astream of ions forming the plasma 68 and a body force which pushes theair and alters the pressure distribution near the vane airfoil pressureand suction sides. The body force applied by the plasma 68 forces theair to pass through the passage between adjacent blades, in the desireddirection of positive flow, reducing flow separations near the airfoilsurfaces and the blade tips. This increases the stability of the fan orcompressor rotor stage and hence the compression system. Plasmagenerators 82, such as for example, shown in FIG. 6, may be mounted onairfoils of some selected fan or compressor stator and rotor stageswhere stall is likely to occur. Alternatively, plasma generators may belocated along the spans of all the compression stage vanes andselectively activated by the instability control system 600 duringengine operation using the engine control system 74 or the electroniccontroller 72. In another exemplary embodiment of the present invention,shown in FIG. 2, plasma actuators 84 are mounted on the IGV flap 32,oriented in a generally spanwise direction. The IGV Flap 32 is movablein order to orient the direction of the airflow entering the first fanrotor 12 a. By energizing the plasma actuator 84, it is possible toextend the range of motion that can be achieved for the IGV flap 32without flow separation. This is especially useful in gas turbine engineapplications where severe inlet flow distortions exist under certaincircumstances.

In other exemplary embodiments of the present invention, it is possibleto have multiple plasma actuators placed at multiple locations in thecompressor casing 50 or the shroud segments 51, in addition to theplasma actuators mounted on stator vane airfoils.

The plasma actuator systems disclosed herein can be operated to effectan increase in the stall margin of the compression systems in the engineby raising the stall line, such as for example shown by the enhancedstall line 113 in FIG. 3. Although it is possible to operate the plasmaactuators continuously during engine operation, it is not necessary tooperate the plasma actuators continuously to improve the stall margin.At normal operating conditions, blade tip vortices and small regions ofreversed flow may exist in the rotor tip region 52. It is firstnecessary to identify the fan or compressor operating points where stallis likely to occur. This can be done by conventional methods of analysisand testing and results can be represented on an operating map, such asfor example, shown in FIG. 3. Referring to FIG. 3, at normal operatingpoints on the operating line 116, for example, the stall margins withrespect to the stall line 112 are adequate and the plasma actuators neednot be turned on. However, as the compression system is throttled suchas for example along the constant speed line 122, or during severe inletair flow distortions, the axial velocity of the air in the compressionsystem stage over the entire stator vane span or rotor blade spandecreases, especially in the tip region 52. This axial velocity drop,coupled with higher pressure rise in the rotor blade tip 46, increasesthe flow over the rotor blade tip and the strength of the tip vortex,creating the conditions for a stall to occur. As the compression systemoperation approaches conditions that are typically near stall the stallline 112, the plasma actuators are turned on. The plasma actuators maybe turned on by the instability control system 600 based on thedetection system 500 input when the measurements and correlationsanalyses from the detection system 500 indicate an onset of aninstability such as a stall or surge. The control system 74 and/or theelectronic controller is set to turn the plasma actuator system on wellbefore the operating points approach the stall line 112 where thecompressor is likely to stall. It is preferable to turn on the plasmaactuators early, well before reaching the stall line 112, since doing sowill increase the absolute throttle margin capability. However, there isno need to expend the power required to run the actuators when thecompressor is operating at healthy, steady-state conditions, such as onthe operating line 116.

Alternatively, instead of operating the plasma actuators 82, 84 in acontinuous mode as described above, the plasma actuators can be operatedin a pulsed mode. In the pulsed mode, some or all of the plasmaactuators 82, 84 are pulsed on and off at (“pulsing”) somepre-determined frequencies. It is known that the tip vortex that leadsto a compressor stall generally has some natural frequencies, somewhatakin to the shedding frequency of a cylinder placed into a flow stream.For a given rotor geometry, these natural frequencies can be calculatedanalytically or measured during tests using unsteady flow sensors. Thesecan be programmed into the operating routines in a FADEC or other enginecontrol systems 74 or the electronic controller 72 for the plasmaactuators. Then, the plasma actuators 82, 84 can be rapidly pulsed onand off by the control system at selected frequencies related, forexample, to the vortex shedding frequencies or the blade passingfrequencies of the various compressor stages. Alternatively, the plasmaactuators can be pulsed on and off at a frequency corresponding to a“multiple” of a vortex shedding frequency or a “multiple” of the bladepassing frequency. The term “multiple”, as used herein, can be anynumber or a fraction and can have values equal to one, greater than oneor less than one. The plasma actuator pulsing can be done in-phase witheach other. Alternatively, the pulsing of the plasma actuators can bedone out-of-phase, at a selected phase angle, with other. The phaseangle may vary between about 0 degree and 180 degrees. It is preferableto pulse the plasma actuators approximately 180 degrees out-of-phasewith the vortex frequency to quickly break down the blade tip vortex asit forms. The plasma actuator phase angle and frequency may selectedbased on the detection system 500 measurements of the tip vortex signalsusing probes mounted in stator stages or near the blade tip as describedpreviously herein.

During engine operation, the mitigation system 300 turns on the plasmagenerator 82, 84 to form the plasma 68 between the first electrode 62and the second electrode 64. An electronic controller 72 may be used tocontrol the plasma generator 82, 84 and the turning on and off of theplasma generator. The electronic controller 72 may also be used tocontrol the operation of the AC power supply 70 that is connected to theelectrodes 62, 64 to supply a high voltage AC potential to theelectrodes 62, 64.

The cold clearance between the annular casing 50 (or the shroud segments51) and blade tips 46 is designed so that the blade tips do not rubagainst the annular casing 50 (or the shroud segments 51) during highpowered operation of the engine, such as, during take-off when the bladedisc and blades expand as a result of high temperature and centrifugalloads. The exemplary embodiments of the plasma actuator systemsillustrated herein are designed and operable to activate the plasmagenerator 82, 84 to form the plasma 68 during conditions of severe inletflow distortions or during engine transients when the operating line israised (see item 114 in FIG. 3) where enhanced stall margins arenecessary to avoid a fan or compressor stall, or during flight regimeswhere clearances 48 have to be controlled such as for example, a cruisecondition of the aircraft being powered by the engine. Other embodimentsof the exemplary plasma actuator systems illustrated herein may be usedin other types of gas turbine engines such as marine or perhapsindustrial gas turbine engines.

The exemplary embodiments of the invention herein can be used in anycompression sections of the engine 10 such as a booster, a low pressurecompressor (LPC), high pressure compressor (HPC) 18 and fan 12 whichhave annular casings or shrouds and rotor blade tips.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The patentable scope of the inventionis defined by the claims, and may include other examples that occur tothose skilled in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral languages of the claims.

1. An instability mitigation system for a rotor, the system comprising:a detection system comprising a sensor capable of generating an inputsignal corresponding to a flow parameter; a correlation processor thatreceives the input signal and a rotor speed signal and generates astability correlation signal; and a mitigation system that facilitatesthe improvement of the stability of the rotor when an onset ofinstability is detected by the detection system wherein the mitigationsystem comprises at least one plasma actuator mounted on a stator stagethat is located in axial proximity to the rotor; a control system forcontrolling the detection system and the mitigation system; wherein thecontrol system comprises a controller that controls an AC potentialapplied to a first electrode and a second electrode of the plasmaactuator; and wherein the controller controls the AC potential bypulsing the AC potential at a selected frequency.
 2. An instabilitymitigation system according to claim 1 wherein the controller controlsthe AC potential by pulsing the AC potential at a frequency that is amultiple of a number blades in the rotor.
 3. An instability mitigationsystem according to claim 1 wherein the plasma actuator is mounted in agenerally span-wise direction on a convex side of a stator vane airfoillocated in the stator stage.
 4. An instability mitigation systemaccording to claim 1 wherein the plasma actuator is mounted in agenerally span-wise direction on a concave side of a stator vane airfoillocated in the stator stage.
 5. An instability mitigation systemaccording to claim 1, wherein the stator stage comprises an inlet guidevane and the plasma actuator is mounted on the inlet guide vane.
 6. Aninstability mitigation system according to claim 5, wherein the plasmaactuator is mounted on a movable inlet guide vane flap.
 7. Aninstability mitigation system according to claim 1, wherein the sensoris spaced radially outwardly and apart from tips of a row of bladesarranged circumferentially on the rotor.
 8. An instability mitigationsystem according to claim 1, wherein the detection system comprises asensor capable of generating an input signal corresponding to a flowparameter at a location near a tip of a blade.
 9. An instabilitymitigation system according to claim 1, wherein the correlationprocessor generates the stability correlation signal based on the inputsignal and the rotor speed signal.
 10. An instability mitigation systemaccording to claim 1, wherein the detection system further comprises aplurality of sensors arranged circumferentially on a static componentaround a centerline axis and spaced radially outwardly and apart fromtips of a row of blades.
 11. An instability mitigation system for arotor, the system comprising: a detection system comprising a sensorcapable of generating an input signal corresponding to a flow parameter;a correlation processor that receives the input signal and a rotor speedsignal and generates a stability correlation signal; and a mitigationsystem that facilitates the improvement of the stability of the rotorwhen an onset of instability is detected by the detection system whereinthe mitigation system comprises at least one plasma actuator mounted ona stator stage that is located in axial proximity to the rotor; acontrol system for controlling the detection system and the mitigationsystem; wherein the control system comprises a controller that controlsan AC potential applied to a first electrode and a second electrode ofthe plasma actuator; and wherein the controller pulses the AC potentialin-phase with a multiple of the vortex shedding frequency at a bladetip.
 12. An instability mitigation system according to claim 1, whereinthe mitigation system further comprises a plurality of plasma actuatorslocated on the stator stage.
 13. An instability mitigation systemaccording to claim 11, wherein the stator stage comprises an inlet guidevane and the plasma actuator is mounted on the inlet guide vane.
 14. Aninstability mitigation system according to claim 13, wherein the plasmaactuator is mounted on a movable inlet guide vane flap.
 15. Aninstability mitigation system according to claim 11, wherein thedetection system comprises a sensor capable of generating an inputsignal corresponding to a flow parameter at a location near a tip of ablade.
 16. An instability mitigation system according to claim 11,wherein the detection system further comprises a plurality of sensorsarranged circumferentially on a static component around a centerlineaxis and spaced radially outwardly and apart from tips of a row ofblades.
 17. An instability mitigation system according to claim 11,wherein the mitigation system further comprises a plurality of plasmaactuators located on the stator stage.
 18. An instability mitigationsystem for a rotor, the system comprising: a detection system comprisinga sensor capable of generating an input signal corresponding to a flowparameter; a correlation processor that receives the input signal and arotor speed signal and generates a stability correlation signal; and amitigation system that facilitates the improvement of the stability ofthe rotor when an onset of instability is detected by the detectionsystem wherein the mitigation system comprises at least one plasmaactuator mounted on a stator stage that is located in axial proximity tothe rotor; a control system for controlling the detection system and themitigation system; wherein the control system comprises a controllerthat controls an AC potential applied to a first electrode and a secondelectrode of the plasma actuator; and wherein the controller pulses theAC potential out-of-phase with a multiple of the vortex sheddingfrequency at a blade tip.
 19. An instability mitigation system accordingto claim 18, wherein the stator stage comprises an inlet guide vane andthe plasma actuator is mounted on the inlet guide vane.
 20. Aninstability mitigation system according to claim 19, wherein the plasmaactuator is mounted on a movable inlet guide vane flap.
 21. Aninstability mitigation system according to claim 18, wherein thedetection system comprises a sensor capable of generating an inputsignal corresponding to a flow parameter at a location near a tip of ablade.
 22. An instability mitigation system according to claim 18,wherein the detection system further comprises a plurality of sensorsarranged circumferentially on a static component around a centerlineaxis and spaced radially outwardly and apart from tips of a row ofblades.
 23. An instability mitigation system according to claim 18,wherein the mitigation system further comprises a plurality of plasmaactuators located on the stator stage.